Multi-speed transmissions requiring drive engaging devices such as clutches or brakes for each speed range or each direction are generally controlled by a hydraulic system. Each speed range or direction requires de-energizing one clutch or brake and energizing another clutch or brake. When a new speed is selected it may be desirable to de-energize both the direction clutch and the de-selected speed clutch. Engagement will generally take place with the new speed clutch energized first and the direction clutch energized later. This sequence may be selected because the direction clutch is of heavier construction and will absorb the shock of engagement with less wear on the working parts. The engagement sequence can of course be reversed with the direction clutch engaging first and the speed clutch engaging secondly if the speed clutch is of sturdier construction, or the speed clutch alone may be energized without de-energizing the directional clutch. Present hydraulic control systems solve this problem by a complex timing and modulating valve arrangement usually involving more than one valve and appropriate valve spools. Further, existing transmissions have generally placed the modulating means between the source pressure and the speed or direction range selectors thereby limiting the flexibility of the modulation valve means to one rate. This single modulating valve of course gives no flexibility to varying the rate of pressure increase or rate of pressure release in individual speed or direction clutches.
The complexity of existing modulating means is illustrated in U.S. Pat. No. 3,198,027 issued to Ramsel, et al and U.S. Pat. No. 3,386,540 issued to Horsch, et al, both patents assigned to the assignee of this invention. Simpler modulating means are illustrated in U.S. Pat. No. 3,091,976 issued to Johnson, et al (also assigned to the assignee of this invention). In all of these patents it is pointed out that the modulating is accomplished by a modulating spool working in concert with at least one other valve spool. It is also pointed out that by positioning the modulating means between the pressure source and the selector valves, the user is limited in the range of modulation available to the various drive engaging devices. Further, because of the complexity of the existing modulating means, when replacement or servicing is necessary to the modulating means the mechanic is presented with a rather complex problem. Finally cost of manufacturing of these rather complex valve arrangements is commensurately high.